Scanning Range Finder

ABSTRACT

A scanning optical range finder in a mobile robot includes an optical emitter circuit, a non-imaging optical element, an optical detector circuit, and a ranging circuit. The non-imaging optical element is arranged to receive optical signals at an entrance aperture thereof responsive to operation of the optical emitter circuit, and to direct the optical signals to an output aperture thereof. The optical detector circuit is configured to receive the optical signals from the output aperture of the non-imaging optical element, and to generate detection signals based on respective phase differences of the optical signals relative to corresponding outputs of the optical emitter circuit. The ranging circuit is configured to calculate a range of a target from the phase differences indicated by the detection signals. Related devices and methods of operation are also discussed.

CLAIM OF PRIORITY

The present application claims the benefit of and priority from U.S.Provisional Patent Application No. 61/899,045, entitled “SCANNING RANGEFINDER,” filed Nov. 1, 2013, the disclosure of which is incorporated byreference herein in its entirety.

TECHNICAL FIELD

The present disclosure relates scanning optical range finders. Inparticular, systems and methods related to scanning optical rangefinders in mobile robots are described herein, such as lawn mowingrobots and remote telepresence robots.

BACKGROUND

Laser range finders may measure a distance to an object based on thepropagation speed of light output from the laser. A typical laserrangefinder operates based on the time of flight principle, by sending alaser pulse in a narrow beam towards a target, and measuring the timetaken by the pulse to be reflected from the target and returned to thesender. The propagation speed of light in air is relatively unaffectedby temperature, humidity, etc. In contrast with sonar, a collimatedlaser beam can allow for measurements to a precisely known location.Such measurements using the speed of light can be very fast, but mayalso require fast detector electronics for accurate measurements.

Laser range finders may also operate based on the indirect time offlight or phase shift method, by sending a laser beam with sinusoidallymodulated optical power to a target. Reflected light (from diffuse orspecular reflections) is monitored, and the phase of the powermodulation is compared with that of the sent light. The phase shiftobtained may be 2π times the time of flight times the modulationfrequency, illustrating that that higher modulation frequencies canresult in a higher spatial resolution.

SUMMARY

According to some embodiments, a scanning optical range finder in amobile robot includes an optical emitter circuit, a non-imaging opticalelement, an optical detector circuit, and a ranging circuit. Thenon-imaging optical element is arranged to receive distinct opticalsignals at an entrance aperture thereof responsive to operation of theoptical emitter circuit, and to direct the optical signals to an outputaperture thereof. The optical detector circuit is configured to receivethe optical signals from the output aperture of the non-imaging opticalelement, and to generate respective detection signals based onrespective phase differences of the optical signals relative tocorresponding outputs of the optical emitter circuit. The rangingcircuit is configured to calculate a plurality of distances from thephase differences indicated by the detection signals, and to identifyone of the plurality of distances as the range of a target.

In some embodiments, the non-imaging optical element may be a compoundparabolic collector element. The robot may further include a rotatableturret including collection optics that are arranged to direct theoptical signal to the compound parabolic collector element responsive torotation of the turret. The compound parabolic collector element mayinclude a parabolic surface defining the entrance and output aperturesat opposing ends thereof, and a flange extending around a periphery ofthe parabolic surface adjacent the entrance aperture thereof. The flangemay have a greater diameter than the entrance aperture and may define alip protruding from the parabolic surface.

In some embodiments, the optical emitter circuit may be configured tosequentially switch the outputs thereof between different frequenciesduring the rotation of the turret.

In some embodiments, the optical emitter circuit may be configured todynamically alter a power level of the outputs thereof during therotation of the turret.

In some embodiments, the optical signals may have different frequencies.The ranging circuit may be configured to calculate the range of thetarget based on a comparison of the plurality of distances indicated bythe respective detection signals.

In some embodiments, the optical detector circuit may include anaveraging detector configured to output the respective detection signalsrepresenting average voltages based on the respective phase differences.The ranging circuit may be configured to calculate, for the respectivedetection signals, a plurality of distances from the average voltagesthereof, and to identify the one of the plurality of distances as therange of the target based on a least common multiple thereof.

In some embodiments, the optical emitter circuit may be configured toprovide a phase shift between the respective outputs thereof.

In some embodiments, the ranging circuit may be configured to determinea time delay between transmission of one of the outputs from the opticalemitter circuit and arrival of a corresponding one of the opticalsignals at the optical detector circuit, and to identify the one of theplurality of distances as the range of the target based on the timedelay.

In some embodiments, the outputs from the optical emitter mayrespectively include a plurality of gated bursts. The ranging circuitmay be configured to determine a time of the arrival of the one of theoptical signals based on a signal strength of a burst thereof exceedinga threshold.

In some embodiments, the ranging circuit may be configured toextrapolate a rising edge of the burst of the one of the optical signalsfrom the signal strength thereof to determine the time of the arrival.For example, the optical detector circuit may be configured to calculatea received signal strength indicator (RSSI) signal indicating the signalstrength and to sample a received signal strength indicator (RSSI) noisefloor to define the threshold, and the ranging circuit may be configuredto extrapolate a time of the rising edge of the burst based on a risetime of the RSSI signal relative to the RSSI noise floor.

In some embodiments, the averaging detector may be configured to outputthe respective detection signals representing the average voltagesresponsive to input signals thereto that are forced to a predeterminedstate.

In some embodiments, the optical emitter circuit may include aprogrammable frequency clock coupled to an optical emitter. The opticalemitter circuit may be configured to vary a frequency of the clock whenthe optical emitter is pointed at a fixed distance calibration target tooutput a plurality of calibration signals therefrom at respectivefrequencies, and may be configured to dynamically adjust the clock toone of the respective frequencies corresponding to one of thecalibration signals having a highest received signal strength indicatedby the optical detector circuit.

In some embodiments, the one of the respective frequencies maycorrespond to a center frequency of a band pass filter included in theoptical detector circuit.

In some embodiments, the respective frequencies may include a currentfrequency of the clock, a frequency greater than the current frequency,and a frequency less than the current frequency. The optical emittercircuit may be configured to set the frequency of the clock duringoperation of the mobile robot.

According to further embodiments, in a method of operating a non-imagingoptical range finder circuit, distinct ranging signals are transmittedfrom an optical emitter circuit, and, in response to the transmission,respective optical signals are received at an optical detector circuitvia a non-imaging optical element. Respective detection signals aregenerated based on respective phase differences of the optical signalsreceived at the optical detector circuit relative to the correspondingranging signals transmitted from the optical emitter circuit. Aplurality of distances are calculated from the phase differencesindicated by the detection signals, and one of the plurality ofdistances is identified as a range of a target.

In some embodiments, the optical signals may have different frequencies.The range of the target may be calculated by comparing the plurality ofdistances indicated by the respective detection signals.

In some embodiments, the respective detection signals may representaverage voltages based on the respective phase differences. In comparingthe respective detection signals, the plurality of distances may becalculated from the average voltages thereof, and the one of theplurality of distances may be identified as the range of the targetbased on a least common multiple thereof.

In some embodiments, the respective ranging signals may be transmittedfrom the optical emitter circuit by sequentially switching between thedifferent frequencies.

In some embodiments, a time delay between transmission of one of theranging signals from the optical emitter circuit and arrival of acorresponding one of the optical signals at the optical detector circuitmay be determined, and the one of the plurality of distances may beidentified as the range of the target based on the time delay.

In some embodiments, the ranging signals from the optical emitter mayrespectively include a plurality of gated bursts, and a time of thearrival of the one of the optical signals may be determined based on asignal strength of a burst thereof exceeding a threshold.

In some embodiments, the time of the arrival may be determined byextrapolating a rising edge of the burst from the signal strengththereof. For example, a received signal strength indicator (RSSI) noisefloor may be sampled to define the threshold, a received signal strengthindicator (RSSI) signal indicating the signal strength may becalculated, and a time of the rising edge of the burst may beextrapolated based on a rise time of the RSSI signal relative to theRSSI noise floor.

According to still further embodiments, an optical range finder circuitincludes an optical emitter circuit, an optical detector circuit, and aranging circuit. The optical emitter circuit is configured to outputrespective ranging signals having different frequencies. The opticaldetector circuit is configured to receive respective optical signalshaving the different frequencies responsive to operation of the opticalemitter circuit and to generate respective detection signals comprisingaverage voltages representing respective phase differences of therespective optical signals relative to the respective ranging signals.The ranging circuit is configured to calculate a range of a target basedon a comparison of the average voltages of the respective detectionsignals.

In some embodiments, the ranging circuit may be configured to calculate,for the respective detection signals, a plurality of distances based onthe average voltages of the respective detection signals, and toidentify one of the plurality of distances as the range of the targetbased on a least common multiple thereof.

In some embodiments, the respective optical signals may be continuouslymodulated and may have respective phase shifts therebetween.

In some embodiments, the optical emitter circuit may be configured tosequentially switch between the respective frequencies to output therespective ranging signals.

In some embodiments, the optical emitter circuit may be configured todynamically alter power levels of the respective ranging signals outputtherefrom.

According to yet further embodiments, a laser range finder for a mobilerobot having a forward speed, may include three of the following: acircuit that sends the laser signal to a distant object; a circuit thatreceives a reflection of the laser signal from the distant object; acircuit that processes the laser signal reflection into an estimatedtime of flight from the robot to the distant object and back at anupdate rate; and a circuit that reduces or eliminates inaccuratereadings based on a comparison of respective range measurements taken ata succession of different positions in space along an approximate butknown locus.

In some embodiments, the update rate may be greater than 8 updates persecond, for example, based on a forward speed (30 cm/s)*desired updates(1)/forward interval (3.6 cm).

It is noted that aspects described herein with respect to someembodiments may be incorporated in different embodiments although notspecifically described relative thereto. That is, all embodiments and/orfeatures of any embodiments can be combined in any way and/orcombination. Moreover, other systems, methods, and/or computer programproducts according to embodiments will be or become apparent to one withskill in the art upon review of the following drawings and detaileddescription. It is intended that all such additional systems, methods,and/or computer program products be included within this description, bewithin the scope of the present disclosure, and be protected by theaccompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are illustrated by way of example andare not limited by the accompanying figures with like referencesindicating like elements.

FIG. 1 is an enlarged cross-sectional view illustrating an optical rangefinder according to some embodiments of the present disclosure.

FIG. 2 is a perspective view illustrating an optical range finderaccording to some embodiments of the present disclosure with its outercover removed.

FIG. 3 is a cross-sectional view illustrating an optical range finderaccording to some embodiments of the present disclosure.

FIG. 4 is an electrical block diagram illustrating an optical rangefinder circuit according to some embodiments of the present disclosure.

FIG. 5A is a graph illustrating phase wraparound in a detection signal.

FIG. 5B is a graph illustrating operations for addressing phasewraparound in a detection signal according to some embodiments of thepresent disclosure.

FIG. 5C is a graph illustrating operations for addressing phasewraparound in a detection signal according to some embodiments of thepresent disclosure.

FIG. 5D is a graph illustrating operations for addressing phasewraparound in a detection signal according to some embodiments of thepresent disclosure.

FIGS. 6A-6B are flowcharts illustrating operations for addressing phasewraparound in a detection signal according to some embodiments of thepresent disclosure.

FIG. 7 is a plot illustrating an output of an optical range finderaccording to some embodiments of the present disclosure after scanning acorner of a room.

FIG. 8 is a plot illustrating an output of an optical range finderaccording to some embodiments of the present disclosure for an outdoorspace having a tree near a building wall.

FIG. 9 is a plot illustrating an output of an optical range finderaccording to some embodiments of the present disclosure for a spacehaving chair legs therein.

FIGS. 10A-10C and 11A-11C are schematic diagrams illustratingcollimating lens assembly configurations for reducing or minimizing theheight of the collimation assembly according to some embodiments of thepresent disclosure.

FIGS. 12A-12D are schematic diagrams illustrating compound parabolicconcentrator (CPC) elements according to some embodiments of the presentdisclosure.

FIGS. 13A and 13B are side and top views, respectively, illustratingvarious positioning of optical range finders on a mobile robot inaccordance with some embodiments of the present disclosure.

FIGS. 14A and 14B are a plot and a graph, respectively, illustratingoperations for improving angular (heading) resolution toretro-reflective beacons in accordance with some embodiments of thepresent disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

Autonomous robots that perform functions such as floor cleaning and lawncutting rely on determining barriers (often user-defined or invisiblebarriers) for confining their motion. Such robots can use localizationsystems based on triangulation to determine the robot position withinthe boundary. In the example of lawnmowers, signals sent between thelawnmower positioned in the property and beacons or environmentalfeatures surrounding the lawn allow the lawnmower to estimate the anglesand the distance by calculating time of flight and using trigonometry tocalculate the robot's current position. Systems and methods related toscanning range finders for use with robotic lawn mowers and otherrobotic devices are described herein. In some examples described herein,a scanning range finder includes a non-imaging optical element. In someadditional embodiments, systems and methods described herein identifyand account for wraparound that can cause a distance beyond wraparoundto appear as a closer distance. Identifying and accounting forwraparound can be beneficial in applications such as lawn mowing wherethe laser range finder can receive signals reflected from objectslocated at a significant distance from the robot.

The invention is described more fully hereinafter with reference to theaccompanying drawings, in which embodiments of the invention are shown.This invention may, however, be embodied in many different forms andshould not be construed as limited to the embodiments set forth herein.Rather, these embodiments are provided so that this disclosure will bethorough and complete, and will fully convey the scope of the inventionto those skilled in the art. In the drawings, the size and relativesizes of layers and regions may be exaggerated for clarity.

It will be understood that when an element or layer is referred to asbeing “on”, “connected to” or “coupled to” another element or layer, itcan be directly on, connected or coupled to the other element or layeror intervening elements or layers may be present. In contrast, when anelement is referred to as being “directly on,” “directly connected to”or “directly coupled to” another element or layer, there are nointervening elements or layers present. As used herein, the term“and/or” includes any and all combinations of one or more of theassociated listed items. Like numbers refer to like elements throughout.

It will be understood that although the terms first and second are usedherein to describe various signals and/or other elements, these signalsand/or elements should not be limited by these terms. These terms areonly used to distinguish one signal or element from another signal orelement. Thus, a first signal or element discussed below could be termeda second signal or element, and similarly, a second signal or elementmay be termed a first signal or element without departing from the scopeof the present disclosure.

Furthermore, relative terms, such as “lower” or “bottom” and “upper” or“top,” may be used herein to describe one element's relationship toanother element as illustrated in the Figures. It will be understoodthat relative terms are intended to encompass different orientations ofthe device in addition to the orientation depicted in the Figures. Forexample, if the device in the Figures is turned over, elements describedas being on the “lower” side of other elements would then be oriented on“upper” sides of the other elements. The exemplary term “lower” cantherefore encompass both an orientation of “lower” and “upper,”depending of the particular orientation of the figure. Similarly, if thedevice in one of the figures is turned over, elements described as“below” or “beneath” other elements would then be oriented “above” theother elements. The exemplary terms “below” or “beneath” can, therefore,encompass both an orientation of above and below.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes,” and/or “including,” when used in thisspecification, specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof.

Embodiments of the invention are described herein with reference tocross-sectional illustrations that are schematic illustrations ofidealized embodiments of the invention. As such, variations from theshapes of the illustrations are to be expected, for example, due tonormal manufacturing tolerances. Thus, the regions illustrated in thefigures are schematic in nature and their shapes are not intended toillustrate the precise shape of a region of a device and are notintended to limit the scope of the invention.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of this disclosure and therelevant art and will not be interpreted in an idealized or overlyformal sense unless expressly so defined herein.

Some embodiments of the present disclosure may arise from realizationthat optical systems typically found in scanning range finders are basedaround imaging optical paths, where the light rays projected from thetarget to the detector map 1:1 such that, if the detector were replacedwith an image sensor, a ‘picture’ could be taken. However, such imagingoptics may be unnecessary in single pixel ranging systems, which insteaddepend upon total light intensity. In addition, imaging-based opticalsystems may have inherent limitations regarding the placement toleranceof the optical elements. Also, the locations of the light rays strikingthe detector may change with a non-bore-sighted optical path, and somedegree of offset between transmit and receive optical paths may benecessary with phase detecting range finders (since crosstalk may not betolerated).

Embodiments of the present disclosure can address the above by providingan optical range finder in a mobile robot that uses a non-imaging oranidolic optical system, instead of imaging-based optical paths. In someembodiments, the anidolic optical system may include a compoundparabolic concentrator or collector (CPC) element that directs lightrays to an optical detector without defining an image of the sourcethereon. In range finders including non-imaging optical systems asdescribed herein, tolerance to optical element misalignment may beincreased, photons that enter the optical system may be gathered anddirected onto the active area of an optical detector at larger angles ofincidence, and the send-receive offset (a non-bore-sighted) optical pathmay be maintained. In general, non-imaging optics are optics designedfor the transfer of light radiation between a source and a target which,unlike traditional imaging optics, do not attempt to form an image ofthe source; instead non-imaging optics provide an optical system foroptical radiative transfer from a source to a target.

Some embodiments of the present disclosure as illustrated in FIGS. 1-3are directed to a mobile robot including a scanning laser range finder100 having a rotatable or spinning turret 110 containing a reflectivepolygonal surface (illustrated as folding mirrors 120) and collectionoptics (including Fresnel lens 115). As shown in the central region ofthe enlarged cross sectional view of FIG. 1, in operation, the turret110 rotates the collection optics 115 and folding mirrors 120, providinga 360 degree scanning mechanism for collection of incident opticalsignals. The terms ‘return’, ‘received’, and ‘reflected’ with referenceto optical signals described herein are used interchangeably to refer toan optical signal that is a reflection of a ranging signal transmittedfrom one or more optical emitters described herein.

Above the turret 110 is the laser and collimation adjustment assembly135, which includes at least one optical emitter (illustrated as a laserdiode 140) and a collimating lens 145. The laser collimation assembly135 is coupled to a pitch/roll mount 155. In FIGS. 1-3, the lasercollimation assembly 135 is illustrated in a vertical configuration, inwhich the laser emitted by the laser diode 140 exits via thetransmission barrel 150 near the top of the turret 110. In someembodiments, the laser range finder 100 may be configured to reduce orminimize the overall height of the system in order to obtain a lowprofile for household, e.g., floor cleaning robot applications. A lowprofile can be useful because, in an 360 degree scanning application,the laser range finder 100 will typically be the topmost element of thestructure (to provide unobstructed visibility of up to 360 degrees), butmay require a significant portion of the system height (either added tothe system height or subtracted from the system internal design volume).As such, FIGS. 10A-10C and FIGS. 11A-11C illustrate alternateconfigurations of the laser assembly 135 that may be used to reduceoverall height. In particular, FIGS. 10A-10C illustrate horizontalconfigurations of the laser collimation assembly, while FIGS. 11A-11Cillustrate alternate vertical configurations of a laser collimationassembly according to embodiments of the present disclosure. Theexamples shown in FIGS. 10A-10C and FIGS. 11A-11C include folded opticalpaths, the laser embedded in the turret 110 with a slip ring, and/or anon-collimated light source 140, among other options for reducing orminimizing the height of the collimation assembly 135 and otherwiseincreasing or maximizing detection of overhanging obstacles.

While discussed above with reference to a topmost position, FIGS.13A-13B illustrate that the scanning laser range finder may be mountedin various and/or multiple positions on a mobile robot 1300. Inparticular, as other sensors and/or user interface elements may competefor the top center location on the robot 1300, the example of FIGS.13A-13B illustrates a scanning laser range finder mounted at twopositions 100′, 100″ at opposite ends (e.g., the front and back) of therobot 1300. The rotation of the scanning laser range finder at the twopositions 100′, 100″ may cover respective scanned areas 1305, 1310around the robot 1300. The scanned areas 1305, 1310 may each cover lessthan 360 degrees due to limitations on rotation that may be imposed bythe respective positions 100′, 100″, but may overlap such that thecombination of the scanned areas 1305, 1310 provides full 360 degreecoverage. Other position(s) of the scanning laser range finder may alsobe possible. More generally, although primarily described herein withreference to a scanning laser range finder 100 mounted at a topmostlocation on a robot or other structure, it will be understood thatscanning laser range finders in accordance with embodiments of thepresent disclosure are not limited to any particular location, and mayeven be located at multiple locations on a robot or other structure.

In some examples, the mobile robot 1300 is an autonomous robot lawnmoweris configured to mow a lawn. The autonomous robot lawnmower moves aboutthe lawn and cuts grass as it is traversing the lawn. The robotlawnmower includes a robot body, a surface treater secured to the robotbody, a sensor system having at least one surface sensor carried by therobot body and responsive to at least one surface characteristic, and adrive system including at least one motorized wheel. The drive system iscarried by the robot body and configured to maneuver the robot lawnmoweracross lawn. In this example, the surface treater is a reciprocatingsymmetrical cutter. The robot body also supports a power source (e.g., abattery) for powering any electrical components of the robot lawnmower,including the drive system, the surface treater, and a navigationsystem. When not mowing the lawn, the robot lawnmower may be docked at abase station or dock. In some examples, the dock includes a chargingsystem for charging the battery housed by the robot body 100. Furtherdetails about the design and operation of the robotic lawn mower can befound, for example, in U.S. patent application Ser. No. 14/512,013 filedon Oct. 10, 2014 and titled “Autonomous Robot Localization,” thecontents of which are hereby incorporated by reference.

For mowing operations the robot lawnmower is placed on the lawn so thatit can mow the lawn which is bounded by perimeter. The robot lawnmoweris constrained to not travel outside of the perimeter. To demark theperimeter, one or more boundary markers can be placed in or around thelawn and/or environments features such as trees and man-made structurescan be used to identify locations surrounding the lawn. The boundarymarkers and/or environmental features reflect a signal generated by thelaser included in the robot lawn mower. A controller in the robot usestrigonometry to estimate the position of the robot based on the signalsreceived and collected by the CPC in the robot. In general, the pose orposition of the robot lawnmower can be determined based on the signalsreflected by the boundary markers and environmental features. Moreparticularly, the robot lawnmower sends a signal (e.g., a laser signal)that is reflected by one of the boundary markers or environmentalfeatures. The robot lawnmower can determine the angle between the robotlawnmower relative to the boundary marker or environmental feature basedon the location at which the signal is received. Additionally, the robotlawnmower can determine the distance between the robot lawnmower and theboundary marker or environmental feature based on the time-of-flightbetween the sending of the signal and the receipt of the reflectedsignal. Thus, based on the information, the robot lawnmower's pose canbe determined by trilaterating based on received time-of-flightinformation (range/heading) from each of the boundary markers orenvironmental features. In general, trilateration is the process ofdetermining absolute or relative locations of points by measurement ofdistances, using the geometry of circles, spheres or triangles. In oneexample, trilaterating can be based on a least squares algorithm usingthe distance/time-of-flight measurements.

Referring again to FIGS. 1-3, below the mirror block 120 in the turret110 is a non-imaging or anidolic optical element, (illustrated as acompound parabolic concentrator (CPC) element 125), which directsphotons to at least one optical detector (illustrated as a photodiode130). The CPC element 125 is a non-imaging element arranged within theturret 110 to receive light from the mirrors 120 during rotation of theturret. The CPC element 125 collects and directs the incident light ontothe photodiode 130 without forming an image of the source thereon, andis illustrated in greater detail in FIGS. 12A-12D. As shown in FIG. 12A(in side view) and FIG. 12B (in perspective view), the CPC element 125is arranged to receive incident light 1200 directed thereto by themirror block 120. The incident light 1200, which includes reflectedoptical signals transmitted by the laser diode 140, is received at theinput or entrance aperture adjacent a flange 1210 and is directed viareflection (which may be surface or total internal reflection by theparabolic surfaces 1215) to the output aperture or surface 1205, whichprovides the light 1200 to the photodiode 130. The CPC element 125allows for increased tolerance of optical misalignment, and can collectlight at larger angles of incidence than some conventional parabolicconcentrator elements.

FIGS. 12C and 12D illustrate an alternate perspective view and a sideview with example dimensions of the CPC element 125, respectively. Asshown in FIGS. 12C and 12D, the flange 1210 is sized and configured tofacilitate manufacture of the CPC element 125 and/or handling of the CPCelement 125 during assembly of the range finder 100. The CPC element 125is formed of an injection molded plastic which provides the benefit ofallowing the CPC element 125 to be manufactured using an injectionmolding process. The CPC element 125 includes two portions—the parabolicsurface 1215 and the flange 1210 which are formed as a unitary piece ofinjection molded plastic. The top and bottom surfaces of the CPC element(e.g., the top surface of the flange 1210 and the bottom surface of theparabolic surface 1215) are both substantially planar. The flange 1210defines a lip or ring protruding from an edge of the parabolic surface1215 and having a diameter greater than that of an adjacent openingdefined by the entrance aperture of the parabolic element. The flange1210 is generally cylindrical in shape with one a planar surface 1220corresponding to an area in which plastic is injected into a mold cavityin fabrication of the CPC element 125. Thus, inclusion of the flange1210 allows the CPC element to more easily be fabricated using aninjection molding process because the molding gate which produces a flatsurface can be positioned on a portion of the CPC element 125 that isnot required to be rotationally symmetric (e.g., it is not part of theparabolic surface 1215 designed to be rotationally symmetric to providea total internal reflection surface).

The greater diameter of the flange 1210 relative to the entranceaperture may allow for ease in removal from the fabrication mold, aswell as for ease of assembly into the range finder (for example, usingpick-and-place methods). For instance, as shown in FIG. 12D, the flange1210 may have an outer diameter of about 5.00 mm and a height of about0.50 mm. In other embodiments, the flange 1210 may have an outerdiameter of between about 3 mm and about 7 mm and a height of betweenabout 0.25 mm and about 0.75 mm. In some embodiments, a portion of theflange 1210 adjacent the entrance aperture of the parabolic surface 1215may be tapered by about 0.05 mm relative to the 5.00 mm diameterthereof. The parabolic surface 1215 may have a height and/or curvaturewhich defines a desired diameter of the output aperture 1205, and orwhich defines a desired angle or slope relative to the outer diameter ofthe flange 1210. For instance, in the example of FIG. 12D, the parabolicsurface 1215 has a height of about 5.50 mm, which defines an angle ofabout 15 degrees relative to the edge of the flange 1210. In someadditional embodiments, the parabolic surface 1215 can have a height ofbetween about 4 mm and about 8 mm and have an angle of between about10-20 degrees relative to the edge of the flange 1210. The height andangle are coordinated to provide a total internal reflection surface.However, although illustrated in FIG. 12D with reference to specificdimensions by way of example, it will be understood that CPC elements asdescribed herein are not limited to such dimensions. In some examples, aratio of the diameter of the top surface of the parabolic surface 1215(e.g., where the parabolic surface 1215 joins the flange 1210) to thebottom surface of the parabolic surface 1215 (e.g., output surface 1205)can be between about 3:1 and about 4:1 (e.g. about 3:1, about 3.5:1,about 4:1). In some examples, a ratio of the diameter of the top surfaceof the flange 1210 (e.g., input surface 1210) to the bottom surface ofthe parabolic surface 1215 (e.g., output surface 1205) can be betweenabout 4:1 and about 6:1 (e.g. about 4:1, about 5:1, about 6:1).

In some embodiments, an indoor mobile robot moving at about 30 cm/s andhaving a platform diameter of about 36 cm may translate less than about1/10th of its diameter between scans. For a translation of scans ofabout 2-5 cm (e.g., 3.6 cm), a desired update rate for the opticaldetector 130 may be about 5-15 Hz, e.g., 8 Hz. Thus, the scan rate maybe about 2-20 Hz, e.g., 5-10 Hz for a floor care robot. If one range isdesired for each degree of arc length, the sample rate at 10 Hz would be3.6 kHz (i.e., 360 samples per rotation of the rotating scanning rangefinder 100). In the embodiments of FIGS. 1-3, the CPC element 125 andphotodiode 130 are fixed or stationary elements within the turret 110,while the mirror block 120 and collection optics 115 rotate with theturret 110 to scan an environment. However, it will be understood thatthe present disclosure is not limited to such a configuration, and inother embodiments the CPC element and/or the photodiode 130 may berotatable.

FIG. 2 is a perspective view illustrating the laser range finder 100with its outer cover removed. FIG. 3 is a cross-sectional view of theentirety of the laser range finder 100. The cross-sectional view of FIG.3 further illustrates the motor 195 on the right, which spins the turret110, and an internal calibration wall 105 that is curved with a sameradius as the turret 110. The calibration wall 105 thus provides areference point that is equidistant at all points during a scan. Theelectronics operating the motor 195 may further include a redundantencoder; if the rotational speed of the turret 110 drops to thresholdlevels, the laser diode 140 may be shut off to limit eye exposure.

As illustrated in FIGS. 1-3, in some embodiments, the laser range finder100 may have a turret height of 8 mm, and a system height of 32 mm. Thelaser range finder 100 may be operational in both dark (e.g., 0 lux) andbright (e.g., sunny outdoor) light environments. In some embodiments,the laser range finder 100 may have a 3.6 kHz sample rate with a 1degree angular heading resolution and 10 Hz scan rate. The embodimentsdepicted in FIGS. 1-3 may have a depth of field measurement range ofabout 2 cm to 4 m, with an accuracy at 4 m of about ±3 cm (which mayvary at different distances depending on the color and/or reflectivityof the target). Some embodiments may not include slip rings, and onlythe optics 115 may spin. In particular embodiments, the laser rangefinder 100 can detect white-colored targets at 4 m within about ±3 cmspecification; at 5 m, this may increase to about ±7 cm. A grey-coloredtarget at the 3 m distance may have an error band of about ±7 cm. The ±3cm specification can be maintained at 2.5 m. Black-colored targets maynot be seen well at 4 m (error band is about ±45 cm). At 1 m, the errorband for black target may be about ±4 cm. Some embodiments may use phaseshift measurements, as described in detail below, and may have a scanangle of about 286 degrees or more (about 226° or more with cover).

FIGS. 7-10 are plots illustrating output data sets from an optical rangefinder according to some embodiments of the present disclosure invarious operational environments. In the data sets illustrated in FIGS.7-10, the plots may illustrate all collected data, including noisy datapoints; however, it will be understood that data points which do nothave sufficient signal strength and/or are otherwise deemed nottrustworthy may be deleted in further embodiments of the presentdisclosure. Also, in FIGS. 7-10, samples/scans are taken over a 0.5degree arc length, such that a scan packet of 360 degrees includes 720samples. As such, if the laser beam ‘jumps’ between objects at differentdistances while one sample is being taken over this arc length, thereported distance may be an average of the two distances. While this maybe a source of error for some samples, it will be understood that astatistical model may be built into an occupancy grid recording. Thus,in a robot mapping application according to embodiments of the presentdisclosure, ‘split samples’ over two distances may not significantlyaffect robot mapping and localization.

In particular, FIG. 7 illustrates the output of the laser range finder100 after scanning the corner of a room. The corner and walls of theroom are indicated by the majority of samples on the plot.

FIG. 8 illustrates the laser range finder output for an outdoor spacehaving a tree near a building wall. The samples of FIG. 8 illustrate thecase where the laser beam intermittently falls on two differentsurfaces, where the circled resulting range reading is an average of thereadings from the two surfaces. However, the circled output datarepresents one point in time; thus, when the laser range finder ismounted to a mobile robot, the circled average data point may fall outof subsequent readings as the robot travels, and the circled data can beidentified as noise that can be ignored.

FIG. 9 illustrates the laser range finder output for a space havingchair legs. The samples of FIG. 9 likewise illustrate the case where thelaser beam intermittently falls on two different surfaces, where thepoints circled indicate the chair legs. FIG. 9 thus illustrates that theimportance of fairly fine angular resolution for accurate navigationthrough ‘clutter’ or other small objects. Such fine angular resolutionmay be provided by embodiments of the present disclosure by using phaseshift measurements, as described in greater detail below.

Pulse-based time of flight (ToF) measurements and phase shiftmeasurements involve several differences. For example, the peak andaverage laser powers are typically much lower in phase shiftmeasurements, as the measurement is made in a different manner. Inparticular, a phase shift-based system's laser is continuouslymodulated, and typically only the transmitted frequency (or frequencies)is observed in the measurement. In this respect, a phase shift-basedsystem relies on frequency domain measurements rather than time domain.In some embodiments of the present disclosure, the scanning laser rangefinder 100 may use a clock gate to perform time of flight measurementsto provide a coarse target range, and may perform phase shiftmeasurements to provide a fine resolution range. That is, the laserrange finder 100 in accordance with embodiments of the presentdisclosure may rely on both time of flight and phase shift inmeasurements.

Further embodiments of the present disclosure may arise from realizationthat, while phase shift-based range detection may be inherently moresensitive than pulse time of flight, due for example, to the noiseimmunity of the detection system afforded by the use of a bandpassfilter and the averaging nature of the measurement acquisition, phaseshift measurement typically employs a continuously modulated transmitterand an averaging detector, rather than direct ToF and a single (or smallnumber of) pulses. However, with a continuously modulated transmitsignal, the phase shift of the return signal ‘wraps around’. This canoccur every 90 degrees with a multiplying or XOR type phase detector(180 degrees round trip). The phase shift method thus introducesambiguity regarding the measured distance, because, with increasingdistance, the phase may vary periodically. As such, for phase-shiftbased detector systems, the phase shift for a long distance can causealiasing to a shorter distance. Further embodiments of the presentdisclosure, which may address this ambiguity, are described below withreference to FIGS. 4, 5A-5D, and 6A-6B.

FIG. 4 is an electrical block diagram of a range finder circuit 400according to embodiments of the present disclosure, and FIG. 6A is aflowchart illustrating operation of a range finder circuit according toembodiments of the present disclosure. As shown in FIG. 4 with referenceto FIG. 6A, a programmable clock source 401 feeds an optical emittercircuit including a driver 402 and at least one optical emitter(illustrated as laser diode 440). The driver 402 drives the diode 440 totransmit an optical signal (also referred to herein as a ranging signal)(Block 600) and sends a reference signal RefClk to the calibration block403. The optical signal, which is received from a target, is collected(for example, by the collection optics 115 of FIGS. 1-3) and directed(for example, by the mirror block 120 and CPC 125 of FIGS. 1-3) to anoptical detector circuit (Block 605). The optical detector circuitincludes at least one optical detector (illustrated as photodiode 430)and elements 403-410 shown in FIG. 4. In operation of the opticaldetector circuit, the electrical signal output from the photodiode 430is converted into a voltage by transimpedance amp (TIA) 404. The outputof the TIA 404 may be differential, or if single-ended the signal fromthe TIA 404 may be amplified by a single-ended to differentialconverting amplifier 405, filtered by a bandpass filter 406, deliveredto a log amplifier 407 for dynamic range compression and signal strengthmeasurement, and transmitted to a phase detector or comparator 408. Thecalibration block 403 also transmits a known signal to the phasedetector 408 for compensation due to thermal drift. Based on the phasedifference between the two input signals, the phase detector 408generates an output signal (also referred to herein as a detectionsignal) (Block 610) indicating an average voltage to a low pass filter409, which is output to buffer 410 for delivery to a host/CPU 411. Thehost/CPU 411 also receives a return signal strength indicator signalRSSI from the log amp 407. The host/CPU 411 controls the calibrationroutines, captures and converts the analog signals to digital data, andculls samples with insufficient signal strength. The host/CPU 411 (alsoreferred to herein as a ranging circuit) calculates the range ordistance to the target (Block 615), which may be proportional to the lowpass filtered output voltage of the phase detector 408.

Particular embodiments of the present disclosure combine the elementsdescribed above to implement a non-imaging scanning range finderincluding a real-time calibration system, a high noise immunity 2-Piphase detector, a photon gathering system, a hybrid phasediscriminating/Time-of-Flight ranging system, dynamic range enhancement,a source-synchronous bandpass filter, and compensation for AmplitudeDependent Timing Errors (ADTEs), as described in detail below.

A real-time calibration system for use in scanning optical range findersin accordance with some embodiments of the present disclosure may use amixed signal scale compensation scheme, in which analog voltage levelsof logic outputs are used to determine range data beginning and endingpoints (voltages for min/max range). By muting the logic inputsappropriately, the output voltage levels can be detected and slope andoffset can be factored into the compensation scheme. This may beaccomplished by forcing the input signals to the phase detector to aknown or predetermined state. In the present embodiment, symmetry in theoutput stage of differential (e.g. LVPECL) logic gates may be takenadvantage of, and swings of both polarities can effectively be inferredfrom one logic state.

The calibration system may further utilize dynamic clock & filterfrequency synchronization, by providing a high Q bandpass filter in thereceiver signal path to reduce or minimize noise bandwidth and improvesignal to noise ratio. This filter can be realized with passivecomponents, and can have significant variation from the ideal frequency.In order to transfer maximum signal, the reference clock signalfrequency should match the frequency of this bandpass filter. Todetermine this frequency, a compensation scheme may be employed wherebythe signal strength is measured while the laser strikes a calibrationtarget (for example, the internal calibration wall 105 shown in FIGS.2-3). After this measurement is made, the reference clock frequency maybe varied up and down from the nominal frequency, and the new nominalfrequency may be set to the frequency which yields the highest signalstrength. This process can be repeated periodically to ensure that thetransmit frequency matches the bandpass filter frequency, regardless oftemperature or other component property variation.

In performing calibration, it is understood that certain componentparameters within the system may have an initial tolerance, and may alsodrift over time (mostly due to temperature, but also due to long-termaging). In order to address these effects and maintain proper rangingcapability, calibration may be performed both as a one-time-programmable(OTP) factory step, and as a continuous on-the-fly (OTF) procedure. Inparticular embodiments of the present disclosure, OTF calibration occursonce per scan (currently 10 Hz) and includes the following operations:

-   -   1. Noise Floor Measurement—The laser diode (element 440 shown in        FIG. 4) is turned off and the Return Signal Strength Indicator        (RSSI) is sampled 64 times and averaged to set a baseline for        the current noise floor of the system. This noise floor can be        used during scanning as a metric to determine if the measurement        is valid (for example, a measurement should be at least 10%        above the noise floor to count as a valid measurement), as well        as to infer the level of ambient light that is present.    -   2. Mute Calibration—During this period, the entire signal chain        up to the phase detector (element 408 shown in FIG. 4) is        electrically disconnected from the phase detector using analog        switches. Circuits internal to the phase detector pull the        inputs to a known state which causes the output to swing to a        maximum value. The output thereby simulates a 180 degree        out-of-phase signal condition. Due to the symmetry of a fully        differential signal path, a totally in phase (0 degree shift)        condition can be inferred. This signal is sampled multiple        times, and the average is stored for the subsequent        measurements. This data can be useful in that the true full        swing of a logic can deviate slightly from nominal. As the full        range of this output may directly correspond to the full        distance or range of measurement for the range finder (before        wraparound), the output swing range should be known precisely to        correlate to distance. For example, at 8 MHz modulation, the        wraparound distance with an XOR type phase detector is 9525 mm.        The nominal output swing of an LVPECL XOR gate is 1.6V, giving a        nominal conversion factor of 9525/1.6=5953 mm/V. However, if        initial tolerance and temperature effects push this output swing        to 1.7V, for example, then the conversion factor would become        9525/1.7=5603 mm/V, resulting in a 350 mm error in every        measurement.    -   3. Offset Calibration—Due to phase shift and propagation delay        inherent to the electrical circuitry in the return signal path,        even if a target could theoretically be measured at 0 mm (i.e.        point blank measurement), the returned signal would not come        back in-phase but would have some nominal phase shift. Though        the nominal value can be calculated, it may be largely unknown        due to component tolerance and temperature shift. Therefore,        embodiments of the present disclosure can measure a known, fixed        distance to determine the phase shift, using the calibration        wall 105 within the range finder 100 (shown in FIGS. 2-3). The        calibration wall 105 is curved with the same radius as the        turret 110, and is therefore equidistant at all points during        measurement. The calibration wall 105 is ranged in the same        manner as any other point in the real world, except that the        resulting data is stored as a constant offset for the next scan        (after which it may be calculated again).

In contrast, OTP calibration may be a factory-specific calibrationoperation, which is performed once to store parameters in a memory (suchas a flash memory module) of the range finder 100, which is to be readupon device start up. The stored parameters are used in matching theelectrical bandpass filter (BPF) center frequency. The BPF includespassive components (inductors and capacitors), and so the initialtolerances may cause the center frequency to vary from the nominal. If ahigh Q filter is used and the laser is modulated at a frequency otherthan the true center frequency, the signal-to-noise ratio (SNR) maysuffer and system performance may be degraded. While components may behand selected and tuned to match the desired center frequency, thismethod is labor intensive and would not be feasible for manufacturing inlarge quantities.

In some embodiments, the BPF center frequency may be determined using aprogrammable frequency clock, which includes a fractional-N phase lockedloop (PLL) that can be varied by a host over a serial connection (I2C orSPI). In this method, the range finder 100 may be implemented with aspecial mode in which the turret 110 is not spinning, but is pointed ata fixed distance target, such as the internal calibration wall 105. Theclock can be swept through a range of frequencies and the RSSI for eachmeasurement can be used to determine which frequency is the actualcenter frequency of the BPF.

If the PLL response is fast enough (i.e., frequency change and lock isfast enough), then this can also be included as an operation in the OTFcalibration, for example, to account for BPF temperature drift. This mayrequire a slight modification to speed up the result, that is, insteadof sweeping a broad range of frequencies, the calibration routine maytake one sample (against the calibration wall 105) at the currentlynominal frequency, and one sample each at a frequency above and afrequency below that nominal. The frequency can thus be set accordinglybased on which of these three measurements indicates the highest RSSI.

In performing calibration, a highly accurate (but somewhat slow) 16-bitSigma-Delta ADC may be used to sample the final stage of the signalchain and provide the range measurement. However, due to the noise inthe system, the lower bits of this measurement may not be useful and mayintroduce error. Accordingly, scanning range finders in accordance withsome embodiments of the present disclosure may utilize faster 12-bit SARconverters, so that multiple samples can be taken on each target andsignal processing methods can be used to help determine the correctrange. In particular embodiments of the present disclosure, targetacquisition includes the following operations:

-   -   1. The laser diode (element 440 shown in FIG. 4) is turned on        and a return signal is received. The sampling does not yet        occur, as there is a built-in timeout period to allow the BPF to        settle.    -   2. The sampling period begins. Phase output and RSSI are sampled        simultaneously for 64 samples. Samples are stored in memory, for        example, via direct memory access (DMA).    -   3. The laser diode 440 is turned off and there is a buffer        switch for the next sample.

This is a ping-pong style buffering scheme. In particular, as operations1 and 2 above are occurring, the range algorithm is applied to theprevious set of samples. The sequence is: input samples into Buffer B,calculate on Buffer A, input samples into Buffer A, calculate on BufferB, and repeat.

-   -   4. Calculate range—The average of both the phase and RSSI        samples are determined. If the average RSSI is less than 10%        above the noise floor, the range is considered to be bad and a        zero is input to the scan buffer. If it is above 10%, the        algorithm continues by applying the conversion determined in the        calibration routine. The system is considered to be linear when        RSSI is above 10%, and the basic equation y=mx+b is utilized,        where y is the range, x is the average ADC count determined from        operation 3 above, m is the slope determined during Mute        Calibration and b is the offset determined from Offset        Calibration. This value is stored in the scan buffer as the        range.    -   5. Once a full scan is complete, the scan buffer is written and        stored in another ping-pong style buffer. Only complete scans        are sent to the host (element 411 shown in FIG. 4), so as Scan A        is recorded, the previous Scan B is sent to the host 411. The        buffers then switch so that Scan A will be sent to the host 411        while Scan B is being recorded.

Scanning optical range finders in accordance with some embodiments ofthe present disclosure may further include a high noise immunity 2-Piphase detector, which may provide immunity from clock duty cycledeviation. For example, conventional analog multipliers and Exclusive-ORtype phase detectors may be ‘1-Pi’ (0-180 degree) phase detectors, whosenon-edge triggered operation may yield high noise immunity (non-latchingbehavior during any given cycle); however, for round trip phasedetection, the effective detection may be only 90 degrees. In addition,such 1-Pi phase detectors may not have the ability to discriminate phasefor signals that are nearly in-phase, or for signals that are nearly 180degrees out of phase if the reference and return signals do not have aexactly 50% duty cycle.

Some embodiments of the present disclosure address the above through theuse of a quadrature or delayed reference clock fed to dual (2-Pi) phasedetectors, which measure phase between the reference signal from theclock and the received signal. This configuration allows fordiscrimination as to whether the target is within the first or secondphase wraparound ‘window’, and also allows for phase determination evenif the reference or received signals do not have an exactly 50% dutycycle. Identifying and accounting for wraparound can be beneficial inapplications such as lawn mowing where the laser range finder canreceive signals reflected from objects located at a significant distancefrom the robot and can be beneficial in applications in whichtelepresence robots might be traveling along a long hallway or in alarge room, such as a large conference room.

Scanning optical range finders in accordance with some embodiments ofthe present disclosure may also include a compact, efficient photongathering system. As noted above with reference to FIGS. 1-3, opticalsystems found in scanning range finders are often based around imagingoptical paths (whereby the light rays projected from the target to theoptical detector map 1:1), which are typically not needed insingle-pixel ranging systems dependent upon total light intensity. Inaddition, imaging optical systems may have inherent limitationsregarding the placement tolerance of the optical elements. Also, thelocations of the rays striking the detector may change with anon-bore-sighted optical path, and a degree of offset between transmitand receive optical paths may be necessary (for example, with phasedetecting range finders, in which crosstalk is not tolerated).

In contrast, some embodiments of the present disclosure employ a morecompact, non-imaging optical system for photon gathering with increasedefficiency. In particular embodiments, instead of imaging-based optics,a compound parabolic concentrator (CPC) 125 (shown in FIGS. 1, 3 and12A-12D) is used to direct light rays to an optical detector withoutdefining an image of the source thereon. The CPC element allows forincreased tolerance to optical element misalignment, gathering anddirecting photons onto the active area of an optical detector at largerangles of incidence, while maintaining the send-receive offset (anon-bore-sighted) optical path.

Scanning optical range finders in accordance with some embodiments ofthe present disclosure may further include a hybrid phasediscriminating/Time-of-Flight (ToF) ranging system. In particular, phaseshift-based range detection may inherently be more sensitive than pulsetime of flight-based range detection, due for example to the noiseimmunity of the detection system afforded by the use of a bandpassfilter and the averaging nature of the measurement acquisition. Phaseshift measurement, however, typically employs a continuously modulatedoptical emitter/transmitter (such as the laser diode 440 of FIG. 4) andan averaging detector (such as the XOR-type phase detector 408 of FIG.4), instead of direct ToF and single (or small number of) pulses. With acontinuously modulated transmit signal, the phase shift of the returnsignal ‘wraps around’ every 90 degrees (180 full round trip with anXOR-type phase detector). As such, for phase-based detector systems, thephase shift for a long distance can cause aliasing to a shorterdistance.

FIG. 5A illustrates the concept of phase wraparound in greater detail.As shown in FIG. 5A, an average voltage va indicated by the phase shiftof a return or received optical signal having a frequency Freq Acorresponds to two distances, d1 and d2, which cannot be distinguished.Thus, a consequence of phase wraparound is that the distance beyondwraparound may appear as a closer distance. The wraparound distance maybe dependent on the laser modulation frequency and the type of phasedetector used. For example, with an XOR type phase detector (such as thephase detector 408 of FIG. 4), only 0-180° phase detection may beperformed without ambiguity (round trip). However, the XOR type phasedetector may allow for better linearity of phase voltage versus distancein the presence of noise than a latching edge-triggered phase detector(e.g., no jitter induced offset), and may be fully differential whichprovides electrical crosstalk reduction.

As such, embodiments of the present disclosure provide several methodsthat allow an XOR-type phase detector to determine or identify in which‘wraparound window’ a target resides. For example, with reference toFIG. 5B and FIG. 6B, phase wraparound may be addressed by emitting asecond signal having a different, second frequency Freq B, which isdeliberately phase shifted relative to the first signal having firstfrequency Freq A. In particular, as shown in FIG. 5B with reference tothe range finder circuit 400 of FIG. 4, a scan is initiated by drivingan optical emitter 440 to output a first signal at Freq A (Block 601).During the scan, the optical emitter 440 is switched to output thesecond signal at Freq B (Block 602). Return signals reflected from atarget at both frequencies Freq A and Freq B are sampled by the opticaldetector 430 (Block 606), from which respective detection signals aregenerated (Block 611). These signals are used by the CPU 411 tocalculate at least two distances for each detection signal (Block 616),representing the closest possible distance for each frequency. Thecalculated distances are compared (Block 617) and, if in agreement, areidentified as the actual range or distance of the target (Block 618). Ifthe calculated distances do not agree (Block 617), a least commonmultiple of the distances indicated by each frequency is calculated bythe CPU 411, for example, by stepping the distances out in a leap-frogmanner (Block 619). This least common multiple of the distancesindicated by each frequency represents the actual range or distance ofthe target.

A combination of two signals having unique signatures can thereby beused to resolve wraparound. More particularly, with reference to FIG.5B, return or received optical signals having different frequencies FreqA and Freq B are sampled and stored in memory as respective detectionsignals. Due to phase wraparound, the average voltage va of the returnsignal at Freq A is indicative of multiple distances d1, d2, d3, and d4.In order to resolve the distance, the sample voltage from the Freq Bmeasurement is used. If, for example, the average voltage was found tobe vb1, this would be indicative of d1. If instead it was found to bevb3, the resolution algorithm would calculate a distance of d1 for va,but find that the distance calculated from vb3 is d3. Since these do notmatch, the algorithm would step through the common multiples of the FreqA and Freq B wraparound distances until it found a common multiple atd3. This then would be chosen as the true distance of the target.

As such, some embodiments of the present disclosure may use signalshaving different frequencies, multiple receive paths, and a comparisonof separate measurements to address phase wraparound. It will beunderstood that wraparound may still occur, albeit at the beat frequencyof the two modulation frequencies. As lower frequencies have longerwraparound distances, wraparound can be pushed to greater than 100 mwith proper selection of frequencies. In selecting frequencies,trade-offs may exist in that closer frequencies may result in a lowerbeat frequency, and therefore, longer wraparound. However, if theselected frequencies of the respective signals are too close together,they may be indistinguishable. For example, if Freq A and Freq B aresimilar, the average voltages va and vb will be nearly the same from onewraparound window to the next. Conversely, if the frequencies of therespective signals are too far apart, they may define two sides of asteep band pass filter BPF, so the farther away from the center, themore of the signals that may be discarded. In some embodimentsfrequencies approximately 200 kHz apart may be utilized. For example,frequencies on either side of 4 MHz, such as 3.9 MHz and 4.1 MHz, may beselected for Freq A and Freq B, respectively.

Also, the optical emitter may be dynamically switched between thediscrete frequencies Freq A and Freq B to emit the multiple rangingsignals sequentially. For example for a scan arc of 0.5 degrees, a 0.25degrees of the arc may correspond to Freq A, and 0.25 degrees of the arcmay correspond to Freq B. However, as the turret 110 is spinning duringthe sampling, it will be understood that in some instances the firstsample set may correspond to a different target than the second sampleset; nevertheless, embodiments of the present invention may provideaccurate distances for many targets. Also, for dynamic switching, thesizes of the sample sets may be reduced for each frequency (as there isonly half as much time to sample), which may result in noisier datasets. In addition, the calculated distances may never truly agree, ascalculation based on sample set may indicate a slightly differentdistance than the other, and this error could be amplified through thewraparound zones.

It will be understood that, although described above primarily withreference to samples at two different frequencies Freq A and Freq B byway of example, more than two discrete frequencies may be utilized byembodiments of the present disclosure in a similar manner. Moreover,while discussed above with reference to sequentially switching betweenthe discrete frequencies, it will be understood that embodiments of thepresent disclosure may utilize multiple optical emitters, each emittingsignals at a different frequency, or a mixer to combine the frequenciesfor one optical emitter to simultaneously emit the signals used in thephase shift-based ranging measurements described herein.

FIG. 5C illustrates further methods for determining or identifying inwhich ‘wraparound window’ a target resides in accordance withembodiments of the present disclosure. Referring to FIG. 5C, phase shiftmeasurements may be combined with pulse time of flight rangingmeasurements by using RSSI (received signal strength) and transmittinggated bursts 505 from the optical emitter to address phase wraparound.In particular, by observing the received signal at the optical detectorand measuring the time delay between the beginning of the transmittedburst and the arrival of the received signal, the ‘gross’ phase shift orround trip time delay can be ascertained. In this way ‘time of flight’is used to determine a coarse or raw ‘phase window’ target distance(i.e., wraparound discrimination), and long-term averaged phase shift isused for precise or fine distance measurement. In some embodiments, asecond signal path may be used for the time-of-flight based measurement.

Combining phase shift and time of flight based ranging measurements todetermine the phase wraparound window in which the target resides, asdiscussed above, may operate on the assumption that the RSSI rise timeafter being filtered by the bandpass filter is fast, or that thebandpass filter is low-Q and the RSSI signal exceeds a noise thresholdlevel within a predictable time period. In particular, because a PINphotodiode based signal chain may result in a noisy RSSI signal, abandpass filter (element 406 in FIG. 4) may be used for noisereduction/rejection; however, this may slow the RSSI rise time, makingdetermination of the target window more difficult.

Referring to FIG. 5D, to address difficulties in determining the targetwindow due to the use of the bandpass filter, embodiments of the presentdisclosure may extrapolate the rising edge or beginning time t₀ of theburst based on the RSSI rise time from sampling the RSSI signal. Inparticular, the RSSI noise floor is sampled, and then the RSSI signal issampled as the clock gate is removed. Based on the RSSI wave shape(envelope), sample amplitude vs. time is used to extrapolate thebeginning t₀ of the rise of the RSSI signal due to the step function ofthe received signal from target. This calculation, while computationallyintensive, is independent of signal strength, as long as the signalstrength is sufficiently above the noise floor. The time t₀ is used todetermine the time delay for the ToF calculation discussed above withreference to FIG. 5C.

Scanning optical range finders in accordance with some embodiments ofthe present disclosure may also provide dynamic range enhancement. Inparticular, as target reflectivity and distance may greatly vary,particularly in outdoor environments, the dynamic range of the receivedsignal can be extremely large. In some embodiments, this large range ofsignal amplitudes may be addressed by passing the received signalthrough a logarithm amplifier (element 407 of FIG. 4). While the dynamicrange of real world signals may exceed that of most log amps, theeffective dynamic range of the system may be enhanced by sampling theRSSI signal early in the gated clock burst. If the signal strengthexceeds a preset value, the transmitted signal modulation index can bedecreased by the CPU (element 411 of FIG. 4). As such, the power levelof the signal emitted by the optical emitter (element 440 of FIG. 4) maybe dynamically changed, which may permit ranging at limits beyond thecapability of the compression system. In an environment including somehighly-reflective targets (such as retro-reflective beacons), theemitted signal power may be reduced for targets havinghigher-reflectivities and increased for targets having lowerreflectivities, and the data sets collected at the different powerlevels may be merged for range calculations. Additionally oralternatively, if the RSSI signal is weak, the modulation index (i.e.,signal strength or extinction ratio) can be increased so thatnon-compliant (poorly reflective) targets can be detected properly.

FIGS. 14A and 14B illustrate operations for improving angular (heading)resolution to retro-reflective beacons in accordance with someembodiments of the present disclosure. A range finder typically reportsranges for a given heading, where the headings are quantized at aparticular granularity. FIG. 14A is a polar plot illustrating aplurality of range samples 1405 received in response to 256 pulses ofthe laser diode 440 with about 1° of synchronization over a scan angleφ, while the laser range finder (LRF) 100 is rotating at about 10 Hz(that is, one revolution in a tenth of a second). Reference designator1401 indicates a retro-reflective beacon within the angle φ scanned bythe laser range finder 100.

FIG. 14B is a graph of amplitude versus time of the laser scanningthrough the environment of FIG. 14A. One reported range from FIG. 14Aoccurs over the time period shown in FIG. 14B, and the samples in FIG.14B represent individual ADC samples (which are received in response toindividual pulses of the laser diode 440, with less than 1° ofsynchronization). As shown in FIG. 14B, as the laser output from thelaser diode 440 sweeps over the beacon 1401, the amplitude of the rangesignal transitions from a relatively noisy level to a relatively stablelevel and back to a relatively noisy level, based on the higherreflectivity of the beacon 1401. Correspondingly, the amplitude of thereceived signal strength (RSSI) transitions from a lower level to ahigher level and back to the lower level.

The range and RSSI signals shown in FIG. 14B illustrate that, while theamplitude of the range signal may require some time to settle due to thepresence of noise, the amplitude of the RSSI signal settles much faster,and sharply transitions between the lower and higher levels/states toindicate the presence of the retro-reflective beacon 1401. When a beacon1401 is present, the time distance between samples may be relativelyfine, allowing for higher precision. In particular, in FIGS. 14A-14B, aseach sample corresponds to about 1 degree, the distance between samplesis less than 1 degree, allowing for improved resolution as to thelocation of the beacon 1401. The example of FIGS. 14A-14B illustrates asingle-frequency sweep or scan; however, the signal strength/powerand/or frequency of the laser diode output may be varied during thescan. The sweep can be performed on initial power-up in an area toverify boundaries, and/or to compare detected boundaries to storedboundaries from a previous scan.

In some embodiments, multiple sweeps or scans may be performed. Forexample, to identify the presence of a beacon 1401, a fast initial sweep(e.g., at a higher rotational frequency) over the angle φ may beperformed. If an abrupt increase in signal strength (or abrupt decreasein noise) is indicated by the initial sweep, a subsequent sweep or scancan be performed at a slower rate to provide higher resolution or focuson an expected retro-reflective beacon location. That is, the subsequentscan may be performed at lower rotational frequency in one or more areasexpected to contain retro-reflective beacons, as indicated from abrupttransitions in the initial sweep.

Based on the expected presence of a beacon, the signal strength for theoutput signal from the optical emitter may be dynamically increased inthe one or more areas indicated from the initial sweep, to ensure thatthe received signal strength when detecting a beacon is above a desiredthreshold. In other words, the power to the laser diode 440 may bealtered at certain portions of a subsequent, lower-speed scan toincrease the RSSI amplitude (and thus, the accuracy of the rangingmeasurement) based on an indication of the presence of the beacon in theinitial, higher-speed scan.

The power to the laser diode 440 can also be dynamically reduced toavoid reflection from diffuse targets at short ranges, which may beconfused with reflection from a retro-reflective target when the returnsignal is strong. In particular, if a signal strength is known for adiffuse target, then a qualification can be made for signal strength vs.distance. For example, for a white target, signal strength vs. distancemay be significantly less than for a retro-reflective target. Thus,thresholds can be set for the received signal strength, and the power tothe optical emitter can be altered accordingly.

Scanning optical range finders in accordance with some embodiments ofthe present disclosure may also include a source-synchronous bandpassfilter. As discussed above with reference to real-time calibration, thesource clock frequency and bandpass filter peak frequency should bematched. While the clock frequency may be dynamically adjusted asdescribed above, the bandpass filter (element 406 of FIG. 4) may beimplemented using a switched capacitor filter in some embodiments. Assuch, if the clock for such a switched capacitor filter is derived fromthe system clock, then the bandpass filter peak frequency can be matchedto the system clock frequency.

Scanning optical range finders in accordance with some embodiments ofthe present disclosure may further include automatic laser alignmentcalibration. In the laser range finding system, the optical path of thetransmit beam should fall within the field of view of the receivingoptical path. As such, the optical detector (such as the photodiode 130of FIG. 1) may be replaced with a plurality of optical detectors (forinstance, arranged in an array), each with their own range detectingcircuitry, which can further reduce the alignment criticality of thetransmit laser. Range detection can thereby occur wherever in the arrayof pixels the laser spot is focused (for example, by the optics 115,mirror block 120, and CPC element 125 of FIG. 1).

In some embodiments, the optical range finder may include an integratedcircuit IC (such as the circuit 400 of FIG. 4) that enables fine controlover the transmitter and acquisition system. In particular, the circuitmay be configured to take samples at up to 1.8 kHz, to sample at 3.6 kHzor greater, to control the emitter bias and modulation current, toprovide an indication of signal strength, to provide some raw indicationof signal strength (raw voltage), to provide some raw indication ofrange (raw voltage), and/or to change the frequency of the opticalemitter. The IC may be combined with the laser range finder optics(transmit and receive) and eye safety management.

Scanning optical range finders in accordance with some embodiment of thepresent disclosure may further allow for compensation of amplitudedependent timing errors (ADTEs). For example, there may benonlinearities and/or other errors in range measurement systems that maynot be accounted for in some of the methods described herein. Inparticular, because the phase delay through the system cannot beperfectly independent of signal amplitude, there may be range errorsthat are dependent on the signal amplitude. Errors of this type can beproblematic, particularly in the receive signal chain due to thesubstantial dynamic ranges involved. However, such errors can also occurin the reference clock, particularly if the reference clock is derivedelectrically rather than optically. These errors are referred to hereinas amplitude dependent timing errors (ADTEs).

If the reference clock is not generated optically (e.g., by monitoringphotons emitted from the laser diode or other light source), someembodiments of the present disclosure may compensate for ADTEs in thetransmit chain using a monitor photodiode that is often included in manylaser diodes, by transmitting a known signal from the laser diode;measuring the optical power of the signal emitted from the laser diode,and at the same time, measuring the signal from the monitor photodiode;and storing the relationship between the monitor photodiode output leveland laser diode output level. Once the relationship between the monitorphotodiode output level and laser diode output level is known, it can beused to set the bias current level (such that the laser diode is abovethreshold) as well as the extinction ratio (i.e., peak power divided bybias power). Once the current through the laser diode is above thresholdand the extinction ratio is maintained, the delay may be uniform and theADTEs may be reduced to an acceptable level.

Some embodiments of the present disclosure may compensate for timingerrors in the transmit chain when using laser diodes, LEDs, or otheroptical emitters that do not include a monitor photodiode, by monitoringthe current through the optical emitter, and using the current as thesignal source for the reference clock, as there may be a negligibledelay between current flow through optical emitter and the photons beingemitted. Also, in the receive signal chain, some embodiments of thepresent disclosure may compensate for ADTEs by measuring (or otherwisecharacterizing) RSSI and phase for various target reflectivities anddistances, and generating a lookup table of these values. During opticaldetector operation, phase and RSSI voltages can be measured; howeverdepending on the compensation scheme employed, the range that isreported may be an interpolated number from this lookup table. Thus theerror for a given signal amplitude and phase delay can be subtractedfrom the original range estimate. The compensation schemes discussedabove can be programmed into the sensor at the factory, and thecompensation required can be determined for each sensor either bymeasurement of each sensor at the factory or for all sensors bycharacterization.

The flowchart and block diagrams in the figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and/or computer program products according tovarious aspects of the present disclosure. In this regard, each block inthe flowchart or block diagrams may represent a module, segment, orportion of code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

The terminology used herein is for the purpose of describing particularaspects only and is not intended to be limiting of the disclosure. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. The corresponding structures,materials, acts, and equivalents of any means or step plus functionelements in the claims below are intended to include any disclosedstructure, material, or act for performing the function in combinationwith other claimed elements as specifically claimed.

The description of the present disclosure has been presented forpurposes of illustration and description, but is not intended to beexhaustive or limited to the disclosure in the form disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of thedisclosure. The aspects of the disclosure herein were chosen anddescribed in order to best explain the principles of the disclosure andthe practical application, and to enable others of ordinary skill in theart to understand the disclosure with various modifications as aresuited to the particular use contemplated.

1. A mobile robot including a scanning optical range finder, comprising:an optical emitter circuit; a non-imaging optical element arranged toreceive distinct optical signals at an entrance aperture thereofresponsive to operation of the optical emitter circuit and to direct theoptical signals to an output aperture thereof; an optical detectorcircuit configured to receive the optical signals from the outputaperture of the non-imaging optical element and to generate respectivedetection signals based on respective phase differences of the opticalsignals relative to corresponding outputs of the optical emittercircuit; and a ranging circuit coupled to the optical detector circuitand configured to calculate a plurality of distances from the phasedifferences indicated by the detection signals, and to identify one ofthe plurality of distances as a range of a target.
 2. The mobile robotof claim 1, wherein the non-imaging optical element comprises a compoundparabolic collector element, and further comprising: a rotatable turretcomprising collection optics that are arranged to direct the opticalsignals to the compound parabolic collector element responsive torotation of the turret.
 3. The mobile robot of claim 2, wherein thecompound parabolic collector element comprises: a parabolic surfacedefining the entrance and output apertures at opposing ends thereof; anda flange extending around a periphery of the parabolic surface adjacentthe entrance aperture thereof, the flange having a greater diameter thanthe entrance aperture and defining a lip protruding from the parabolicsurface.
 4. The mobile robot of claim 2, wherein the optical emittercircuit is configured to sequentially switch the outputs thereof betweendifferent frequencies during the rotation of the turret.
 5. The mobilerobot of claim 2, wherein the optical emitter circuit is configured todynamically alter power levels of the outputs thereof during therotation of the turret.
 6. The mobile robot of claim 1, wherein: thedistinct optical signals have different frequencies; and the rangingcircuit is configured to calculate the range of the target based on acomparison of the plurality of distances indicated by the respectivedetection signals.
 7. The mobile robot of claim 6, wherein: the opticaldetector circuit comprises an averaging detector configured to outputthe respective detection signals representing average voltages based onthe respective phase differences; and the ranging circuit is configuredto calculate, for the respective detection signals, the plurality ofdistances from the average voltages thereof, and to identify the one ofthe plurality of distances as the range of the target based on a leastcommon multiple thereof.
 8. The mobile robot of claim 6, wherein theoptical emitter circuit is configured to provide a phase shift betweenthe respective outputs thereof.
 9. The mobile robot of claim 1, whereinthe ranging circuit is configured to determine a time delay betweentransmission of one of the outputs from the optical emitter circuit andarrival of a corresponding one of the optical signals at the opticaldetector circuit, and to identify the one of the plurality of distancesas the range of the target based on the time delay.
 10. The mobile robotof claim 9, wherein the outputs from the optical emitter respectivelycomprise a plurality of gated bursts, and wherein the ranging circuit isconfigured to determine a time of the arrival of the one of the opticalsignals based on a signal strength of a burst thereof exceeding athreshold.
 11. The mobile robot of claim 9, wherein the ranging circuitis configured to extrapolate a rising edge of the burst of the one ofthe optical signals from the signal strength thereof to determine thetime of the arrival.
 12. The mobile robot of claim 11, wherein: theoptical detector circuit is configured to calculate a received signalstrength indicator (RSSI) signal indicating the signal strength and tosample a received signal strength indicator (RSSI) noise floor to definethe threshold; and the ranging circuit is configured to extrapolate atime of the rising edge of the burst based on a rise time of the RSSIsignal relative to the RSSI noise floor.
 13. The mobile robot of claim7, wherein the averaging detector is configured to output the respectivedetection signals representing the average voltages responsive to inputsignals thereto that are forced to a predetermined state.
 14. The mobilerobot of claim 2, wherein the optical emitter circuit comprises aprogrammable frequency clock coupled to an optical emitter, and whereinthe optical emitter circuit is configured to vary a frequency of theclock when the optical emitter is pointed at a fixed distancecalibration target to output a plurality of calibration signalstherefrom at respective frequencies, and is configured to dynamicallyadjust the clock to one of the respective frequencies corresponding toone of the calibration signals having a highest received signal strengthindicated by the optical detector circuit.
 15. The mobile robot of claim14, wherein the respective frequencies comprise a current frequency ofthe clock, a frequency greater than the current frequency, and afrequency less than the current frequency, and wherein the opticalemitter circuit is configured to set the frequency of the clock duringoperation of the mobile robot.
 16. A method of operating a non-imagingoptical range finder circuit, the method comprising: transmittingdistinct ranging signals from an optical emitter circuit; receiving, atan optical detector circuit, respective optical signals from anon-imaging optical element responsive to the transmitting; generatingrespective detection signals based on respective phase differences ofthe respective optical signals received at the optical detector circuitrelative to the corresponding ranging signals transmitted from theoptical emitter circuit; calculating plurality of distances from thephase differences indicated by the detection signals; and identifyingone of the plurality of distances as a range of a target.
 17. The methodclaim 16, wherein the respective optical signals have differentfrequencies, and wherein the identifying comprises comparing theplurality of distances indicated by the respective detection signals.18. The method of claim 17, wherein the respective detection signalsrepresent average voltages based on the respective phase differences,and wherein the comparing comprises: calculating, for the respectivedetection signals, the plurality of distances from the average voltagesthereof; and identifying the one of the plurality of distances as therange of the target based on a least common multiple thereof.
 19. Themethod of claim 18, further comprising: sequentially switching betweenthe different frequencies to transmit the respective ranging signalsfrom the optical emitter circuit.
 20. The method of claim 16, furthercomprising: determining a time delay between transmission of one of theranging signals from the optical emitter circuit and arrival of acorresponding one of the optical signals at the optical detectorcircuit; and identifying the one of the plurality of distances as therange of the target based on the time delay.
 21. The method of claim 20,wherein the ranging signals from the optical emitter respectivelycomprise a plurality of gated bursts, and further comprising:determining a time of the arrival of the one of the optical signalsbased on a signal strength of a burst thereof exceeding a threshold. 22.The method of claim 21, wherein determining the time of the arrival ofthe one of the optical signals comprises: extrapolating a rising edge ofthe burst from the signal strength thereof.
 23. The method of claim 22,wherein extrapolating comprises: sampling a received signal strengthindicator (RSSI) noise floor to define the threshold; calculating areceived signal strength indicator (RSSI) signal indicating the signalstrength; and extrapolating a time of the rising edge of the burst basedon a rise time of the RSSI signal relative to the RSSI noise floor. 24.An optical range finder circuit, comprising: an optical emitter circuitconfigured to output respective ranging signals having differentfrequencies; an optical detector circuit configured to receiverespective optical signals having the different frequencies responsiveto operation of the optical emitter circuit and to generate respectivedetection signals comprising average voltages representing respectivephase differences of the respective optical signals relative to therespective ranging signals; and a ranging circuit configured tocalculate a range of a target based on a comparison of the averagevoltages of the respective detection signals.
 25. The optical rangefinder circuit of claim 24, wherein the ranging circuit is configured tocalculate, for the respective detection signals, a plurality ofdistances based on the average voltages of the respective detectionsignals, and to identify one of the plurality of distances as the rangeof the target based on a least common multiple thereof.
 26. The opticalrange finder circuit of claim 25, wherein the respective optical signalsare continuously modulated and have respective phase shiftstherebetween.
 27. The optical range finder circuit of claim 26, whereinthe optical emitter circuit is configured to sequentially switch betweenthe respective frequencies to output the respective ranging signals. 28.The optical range finder circuit of claim 26, wherein the opticalemitter circuit is configured to dynamically alter power levels of therespective ranging signals output therefrom.
 29. A method of mowing anarea with an autonomous mowing robot, the method comprising: controllingthe mowing robot to autonomously traverse the area bounded by apredetermined boundary, including altering direction of the mowing robotat or near the boundary so as to redirect the robot back into thebounded area; and determining a pose of the mowing robot relative tolocations of at least two retro-reflective beacons within the boundedarea, wherein determining the pose of the mowing robot relative to thelocations of the at least two retro-reflective beacons comprises:pulsing a rotating laser range finder during a planar sweep about anaxis of rotation; monitoring an amplitude of a received signal strength(RSSI) responsive to the pulsing; identifying a planar sweep angle atwhich the RSSI sharply transitions from a low level state to a highlevel state thereby indicating the presence of one of the at least tworetro-reflective beacons; and calculating a heading from the mowingrobot to the at least two retro-reflective beacons.
 30. The method ofclaim 29, wherein the laser range finder fires 256 pulses of a laserdiode with about 1 degree of synchronization during a rotational sweep.31. The method of claim 29, wherein the laser range finder rotates at aspeed of at least 10 Hz.
 32. The method of claim 29, wherein the laserrange finder alters a frequency or power of a laser diode output duringthe planar sweep.
 33. The method of claim 29, wherein the laser rangefinder comprises: an optical emitter circuit; a non-imaging opticalelement arranged to receive distinct optical signals at an entranceaperture thereof responsive to operation of the optical emitter circuitand to direct the optical signals to an output aperture thereof; anoptical detector circuit configured to receive the optical signals fromthe output aperture of the non-imaging optical element and to generaterespective detection signals based on respective phase differences ofthe optical signals relative to corresponding outputs of the opticalemitter circuit; and a ranging circuit coupled to the optical detectorcircuit and configured to calculate a plurality of distances from thephase differences indicated by the detection signals, and to identifyone of the plurality of distances as a range of a target.
 34. The methodof claim 33, wherein the non-imaging optical element comprises acompound parabolic collector element, and further comprising: arotatable turret configured to rotate about the axis of rotation andcomprising collection optics that are arranged to direct the opticalsignals to the compound parabolic collector element responsive torotation of the turret during the planar sweep.
 35. The mobile robot ofclaim 34, wherein the compound parabolic collector element comprises: aparabolic surface defining the entrance and output apertures at opposingends thereof; and a flange extending around a periphery of the parabolicsurface adjacent the entrance aperture thereof, the flange having agreater diameter than the entrance aperture and defining a lipprotruding from the parabolic surface.
 36. The method of claim 29,further comprising changing a rotation speed of the laser range finder,wherein, during a lower-speed scan, power to a laser diode from anoptical emitter circuit of the laser range finder is configured to bedynamically increased at the locations of the at least tworetro-reflective beacons, thereby increasing RSSI amplitude based onidentifying the locations of the at least two retro-reflective beaconsin an initial, higher-speed scan.
 37. The method of claim 29, furthercomprising reducing power to a laser diode of the laser range finder toavoid reflection from diffuse targets at short ranges, the diffusetargets having known signal strengths.
 38. The method of claim 29,wherein the retro-reflective beacons are encoded with uniqueidentification markers and the distances between the retro-reflectivebeacons are known.
 39. The method of claim 29, wherein the at least tworetro-reflective beacons comprises three retro-reflective beacons, andwherein determining the pose comprises: performing triangulation to thethree retro-reflective beacons to calculate the pose within the boundedarea.
 40. The method of claim 29, wherein coordinates of theretro-reflective beacons relative to the predetermined boundary areknown and determining the pose based on the retro-reflective beaconsfurther comprises determining the pose relative to the predeterminedboundary of the area.